Method and an apparatus for aligning first and second objects with each other

ABSTRACT

A distance between a mask and a wafer is set such that exposure light beams emerged from the mask are converged by the projection lens to be focused on the wafer. Two light beams with same wavelength of the alignment light beam emerge from a first point (b) which is located far away from the mask, and are focused on the wafer or neighborhood of it by the projection lens. Two mask marks of diffraction gratings are formed on the mask and spaced at a predetermined distance from each other. When the alignment light beams are applied to the mask marks, two diffracted light beams of predetermined order emerge individually from the mask marks in such a manner that the respective optical axes of the two diffracted light beams, which are directed oppositely to of the diffracted light beams, intersect each other on the first point. Thus, the diffracted light beams advance as if the diffracted light beams were the two light beams emerging from the first point. Therefore, the two diffracted light beams can be focused on the wafer or neighborhood of it or can be incident on a wafer mark which is a diffraction grating. Thus, rediffracted light beams emerge from the wafer mark and are detected, so that the mask and the wafer are aligned with each other. Accordingly, the alignment can be performed, despite a great diffraction between the wave-lengths of the exposure light beam and the alignment light beam.

This is a continuation of application Ser. No. 07/251,842, filed onSept. 30, 1988, now U.S. Pat. No. 4,902,133.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and an apparatus for aligningfirst and second objects with each other, and more particularly, to amethod and an apparatus for aligning a mask and a wafer with each otherduring a projection/exposure process in the manufacture of semiconductordevices.

2. Description of the Related Art

In a projection/exposure process in the manufacture of a semiconductordevice, an exposure light beam emitted from light source 1 is applied toa circuit pattern previously formed on mask 2, as shown in FIG. 1. Animage of the circuit pattern is projected on wafer 4 after being reducedin size by means of projection lens 3. Thereupon, a resist of wafer 4 isexposed, so that the pattern image is transferred to wafer 4.

In order to transfer the image of the circuit pattern accurately to apredetermined portion of the wafer, the mask and wafer must be alignedwith each other before the exposure light beam is applied to the mask.The TTL (through the lens) method is a major aligning method for thispurpose. This method is characterized in that an alignment light beam,which has a wavelength different from that of the exposure light beam,is transmitted through projection lens 3. A method using two diffractiongratings is stated in some documents (by G. Dubroeucq, 1980, ME; W. R.Trutna Jr., 1984, SPIE), as an example of the TTL method. As shown inFIG. 2, diffraction gratings 5 and 6 are formed on mask 2 and wafer 4,respectively. An alignment light beam emitted from alignment lightsource (laser light source) 7 is diffracted along a path fromdiffraction grating 6 of the wafer to diffraction grating 5 of the mask.The intensity of the diffracted light beam is detected by means ofdetector 8. Since the diffracted light beam carries information ondislocation between the mask and wafer, the position of the waferrelative to the mask is detected as the intensity of the diffractedlight beam changes.

It is to be desired that the wire of the circuit pattern should be asthin as possible, that is, resolution R=∝λ/NA should be minimized (λ:wavelength of the exposure light; NA=sinα, where α is half the angle atwhich the exposure light beam is converged on the wafer). Resolution Rcan be lessened by widening angle α or reducing λ. Due to structuralrestrictions on the projection lens, however, half-angle α cannot beunlimitedly increased. It is advisable, therefore, to reduce wavelengthλ of the exposure light beam. Presently, a g-line light beam (436 nm) isutilized as the exposure light beam. For a thinner circuit pattern wire,however, an i-line light beam (365 nm) or Krf excimer laser beam (248nm) is expected to be used as the exposure light beam in the future.

The resist of wafer 4 is sensitive to a light beam with a wavelength of500 nm or less. Accordingly, a light beam with a wavelength exceeding500 nm is used as the alignment light beam, in order to avoid affectingthe resist. Currently, an He-Ne laser beam of 633-nm wavelength is themost prevalent light beam for the purpose. Even at present, therefore,the exposure light beam and the alignment light beam have differentwavelengths. The difference between the two wavelengths, however, isexpected to be increased in the future.

Meanwhile, the image of the circuit pattern should be formed focused onthe wafer for accurate exposure thereon. Thus, the distance between themask and wafer is set so that the exposure light beam from the mask canbe converged by the projection lens to be focused on the wafer. In otherwords, the aberration of the projection lens is adjusted so as to beminimized only for the exposure light beam, that is, the projection lenshas chromatic aberration for light beams of any other wavelengths thanthat of the exposure light beam.

In aligning the mask and wafer with each other, therefore, thediffracted alignment light beam from the mask cannot be focused on thewafer, and instead, is focused on a point at distance d from the wafer,as shown in FIG. 2. If a g-line beam (436 nm) is used as the exposurelight beam, the distance between the mask and wafer ranges from about600 mm to 800 mm, while distance d is only scores of millimeters.

Conventionally, ordinary engineers believes that the sensitivity ofdiffracted light beams to be detected is too low for a mask and a waferto be aligned accurately with each other, unless the diffractedalignment light beam is focused on a mask mark. Therefore, prior artaligning apparatuses are provided with means for correcting the lengthof the optical length of the diffracted alignment light beam, as shownin FIG. 2. More specifically, return mirrors 9 are disposed in themiddle of the path of the diffracted alignment light beam. The opticalpath of the diffracted alignment light beam is extended by the distancefor which the diffracted alignment light beam passes between mirrors 9,so that the diffracted alignment light beam from the mask can be focusedon the wafer. If the aligning apparatus is provided with such correctionmeans, however, the apparatus will inevitably be complicated inconstruction.

If a Krf excimer laser beam, whose wavelength is extremely short (248nm), is used as the exposure light beam, moreover, the differencebetween the wavelengths of the exposure light beam and the alignmentlight beam is very large. Therefore, the diffracted alignment light beamis focused on a point at distance D (several thousands of millimeters)from the wafer, as shown in FIG. 2. In this case, the return mirrorsmust be positively increased in size or complicated in construction, inorder to correct the length of the optical path of the alignment lightbeam. Practically, therefore, it is impossible to correct to the opticalpath length by means of the return mirrors. Thus, if the wavelength ofthe exposure light beam is very short (i.e., if there is a greatdifference between the wavelengths of the exposure light beam and thealignment light beam), the mask and wafer conventionally cannot bealigned with each other.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and anapparatus for highly accurately aligning first and second objects witheach other by means of a simple arrangement, in a system such that aprojection lens is interposed between the first and second objects,whereby a light beam with a wavelength different from that of analignment light beam is transmitted through the projection lens to beincident upon the second object, thereby forming an image of the firstobject thereon.

More specifically, the object of the invention is to provide a methodand an apparatus capable of accurately aligning a mask and a wafer witheach other by means of a simple arrangement, despite a great differencebetween the wavelengths of an exposure light beam and an alignment lightbeam.

According to the present invention, there is provided a method foraligning first and second objects with each other, which objects aremoved relative to each other and in parallel, so as to be aligned, aprojection lens being disposed between the first and second objects, afirst mark formed on the first object, the first mark including adiffraction grating region having two diffraction points, each capableof diffracting a light beam applied thereto, the two diffraction pointsspaced at a predetermined distance from each other, a second mark formedon the second objects, the second mark including a diffraction gratingregion, the method comprising steps of:

directing an alignment light beam emitted from a light source to thefirst mark, the alignment light beam diffracted by the two diffractionpoints of the first mark, so that two diffracted light beams orpredetermined orders emerge individually from the two diffraction pointsin such a manner that the respective optical axes of the twopredetermined-order diffracted light beams, which are directed oppositeto the advancing direction of the diffracted light beams, intersect eachother on a first intersection point at a predetermined distance from thefirst mark;

transferring the two predetermined-order diffracted light beams throughthe projection lens toward the second mark, so that the two diffractedlight beams are converged by the projection lens and are incident on thediffraction grating region of the second mark in such a manner that therespective optical axes of the two diffracted light beams, which aredirected to the advancing direction of the diffracted light beams,intersect each other on a second intersection point at a predetermineddistance (=d₁ ≧0) from the second mark, whereby the two diffracted lightbeams are diffracted by the diffraction grating region of the secondmark, and two re-diffracted light beams of predetermined orders emergefrom the diffraction grating region of the second objects;

detecting the predetermined-order re-diffracted light beams andgenerating a detection signal; and

adjusting the first and second objects relative to each other inresponse to the detection signal, thereby aligning the first and secondobject with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a projection/exposure unit;

FIG. 2 is a schematic view of a prior art aligning apparatus used in theprojection/exposure unit shown in FIG. 1;

FIG. 3 is a schematic view of an aligning apparatus according to apreliminary invention precedent to the present invention;

FIG. 4 is a schematic view of an aligning apparatus according to a firstembodiment of the present invention;

FIG. 5 is a diagram for illustrating the principle of the aligningapparatus of the invention shown in FIG. 4;

FIG. 6 is a schematic view of an aligning apparatus according to asecond embodiment of the present invention;

FIG. 6A is a partial enlarged view of the aligning apparatus shown inFIG. 6;

FIG. 7 is a perspective view of an aligning apparatus according to athird embodiment of the present invention;

FIGS. 8A, 8B, and 8C are plan views individually showing alignmentmarks; and

FIG. 9 is a plan view of a combination of a mask and a wafer, showing amodified arrangement of the alignment marks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has conventionally been believed that a diffracted alignment lightbeam emitted from one alignment mark must be focused on the otheralignment mark. However, the inventors hereof have found that this isnot indispensable. Prior to a description of the present invention, apreliminary invention based on this finding will be explained.

As shown in FIG. 3, an alignment light beam emitted from laser 23 isapplied to wafer mark (diffraction grating) 22 via mirror 25 andprojection lens 13. As a result, n-order diffracted light beams emergefrom mark 22. If the exposure light beam is a g-line (436 nm), two±1-order diffracted light beams of the n-order diffracted light beamsare converged by lens 13 to be focused on a point at distance d (severaltens of millimeters) from mask mark (diffraction grating) 21. Theselight beams are spaced at a predetermined distance when they areincident upon mark 21. The two ±1-order diffracted light beams areindividually transmitted through mark 21 to be diffracted thereby, sothat two ±1-order re-diffracted light beams emerge. These re-diffractedlight beams are transferred to detector 26 via mirror 27 and prism 28.In the meantime, the diffracted light beams are superposed to interferewith each other, thus forming an interference light beam, that is, aninterference fringe. The intensity change of the interference light beamis detected by means of detector 26. The ±1-order diffracted light beamsdiffracted by wafer mark 22 carry position information based on thetheir phase change. The ±1-order re-diffracted light beams diffracted bymask mark 21 carry information on the respective positions a mask and awafer, based on their phase change. Accordingly, the interference lightbeam carries information on the mask and wafer positions. Thus, therelative positions of the mask and wafer can be determined by detectingthe intensity change of the interference light beam. The mask and waferare aligned with each other in accordance with the result of thedetection.

Even though the ±1-order diffracted light beams diffracted by wafer mark22 are not focused on mask mark 21, therefore, the mask and wafer can bealigned with each other.

When using a Krf excimer laser (248 nm) as the exposure light beam,however, its wavelength is considerably different from that of thealignment light beam (633 nm). As shown in FIG. 3, therefore, thealignment light beam is focused on point b at distance D (severalthousands of millimeters) from mask mark 21. Accordingly, the two±1-order diffracted light beams diffracted by wafer mark 22 are spacedat so long a distance from each other, at the position corresponding tomask mark 21, that they cannot be incident upon mark 21. If the maskmark is increased in size, however, the detection of the diffractedlight beams is liable to err. Thus, even according to the preliminaryinvention by the inventors hereof, the mask and wafer cannot be alignedwith each other if the wavelength of the exposure light beam isextremely short.

Thereupon, the inventors hereof have completed the present invention asdescribed below. According to this invention, the mask and wafer can bealigned with high accuracy even if there is a great difference betweenthe respective wavelengths of the exposure light beam and the alignmentlight beam.

Referring now to FIGS. 4 and 5, a first embodiment of the presentinvention will be described.

An aligning apparatus is constructed as follows. Two mask marks 41-1 and41-2, each composed of a diffraction grating, is formed on mask 11.These marks are spaced at a shown predetermined distance from eachother. Each mask mark may be a one- or two-dimensional or checkereddiffraction grating. One wafer mark 42, composed of a diffractiongrating, is formed on wafer 12. Mark 42 may also be a one- ortwo-dimensional or checkered diffraction grating.

In the aligning apparatus according to this embodiment, in contrast withthe case of the conventional apparatus, the optical path of thealignment light beam extends from laser 43 to detector 47 via mask marks41-1 and 41-2, projection lens 13, and wafer mark 42, in the ordernamed. Thus, the alignment light beam emitted from laser 43 is splitinto two light beams by mirror 44-1 and prism 44-2, and the split beamsare transferred to marks 41-1 and 41-2, individually. Thereupon, the twolight beams are transmitted individually through mask marks 41-1 and41-2 to be diffracted thereby, so that two n-order (n=0, ±1, ...)diffracted light beams emerge. The two diffracted light beams of thepredetermined orders are transferred to wafer mark 42 through projectionlens 13. Then, these light beams are individually reflected by mark 42to be diffracted thereby, so that two n-order re-diffracted light beamsemerge. The re-diffracted light beams of the predetermined orders aretransferred to detector 47 via mirror 45, lens 46, mirror 53-1 and prism53-2. Then, re-diffracted light beams are converted into detectionsignals by detector 47. These detection signals are processed by meansof signal processing unit 48. In response to an output signal from unit48, the position of the mask or the wafer is adjusted by means ofposition adjusting unit 49.

Comparing FIGS. 3, 4 and 5, the principle of the present invention willnow be described.

If the exposure light beam is a Krf excimer laser, as shown in FIG. 3,the diffracted light beams from wafer mark 22 are focused on point b atdistance D (several thousands of millimeters) from the mask, asmentioned before. When two light beams having the same wavelength as thealignment light beam emerge from point c at distance dl (several tens ofmillimeters) from the wafer, therefore, they are focused on point b atdistance d₂ ##EQU1## from the mask, as shown in FIG. 5 (β is the inversemagnification of the projection lens for the alignment light beam).

Here let it be supposed that imaginary mask 11-1 is disposed at point b.If two light beams emerge from point b on mask 11-1, they are focused onpoint c. In the present invention, two mask marks 41-1 and 41-2 aresituated on the respective optical paths of these light beams,individually. Thus, marks 41-1 and 41-2 are spaced at a predetermineddistance so that the two light beams pass them separately. Therefore, ifthe respective optical axes of the two diffracted light beams of thepredetermined orders, diffracted by mask marks 41-1 and 41-2, are set soas to intersect each other on point b on imaginary mask 11-1, thediffracted light beams are transferred along the optical paths of thelight beams from point b to wafer mark 42. The two predetermined orderdiffracted light beams include a -1-order diffracted light beamdiffracted by mask mark 41-1 and a 1-order diffracted light beamdiffracted by mask mark 41-2.

Wafer mark 42 is also situated on the respective optical paths of thetwo light beams emerging from point b. Accordingly, the two ±1-orderdiffracted light beams from the mask marks are spaced at a predetermineddistance when they are incident upon mark 42. These diffracted lightbeams are reflected by the wafer mark to be diffracted thereby, so thattwo n-order re-diffracted light beams emerge. Thus, ±1-orderre-diffracted light beams of two n-order re-diffracted light beamsemerge at right angles to the wafer mark. The re-diffracted light beamsare converged by projection lens 13, are reflected by mirror 45, andthen advance in parallel by means of lens 46. The re-diffracted lightbeams are transferred to detector 47 via mirror 53-1 and prism 53-2. Asin the case of FIG. 3, the diffracted light beams interfere with eachother, thus forming an interference light beam, that is, an interferencefringe. The intensity change of the interference light beam is detectedby means of detector 47. This intensity corresponds to dislocationbetween the mask and wafer. Based on the detection result of theintensity change of the interference light beam, therefore, positionadjusting unit 49 adjusts the position of the mask or wafer.

Thus, according to the first embodiment, the two ±1-order diffractedlight beams diffracted by the two mask marks are supposed to be lightbeams emerging from imaginary mask 11-1. Accordingly, the diffractedlight beams from mask marks are focused on point c at a relatively shortdistance from a focal surface of the wafer, so that they can be incidentupon the wafer mark. Even if the wavelength of the exposure light beamis extremely short, as in the case of the Krf excimer laser, forexample, the mask and wafer can be aligned with each other, based on theabove-described principle of the present invention. Unlike the case ofthe conventional arrangement, moreover, return mirrors for correctingthe optical paths of the diffracted light beams need not be disposedbetween the mask and wafer. Thus, the arrangement between the mask andwafer is simplified.

Referring now to FIG. 6, a second embodiment of the present inventionwill be described.

As shown in FIG. 6, the second embodiment resembles the first embodimentin that two ±1-order diffracted light beams from two mask marks aretransferred to wafer mark 42 as if they were light beams emerging fromimaginary mask 11-1. The second embodiment, however, differs from thefirst embodiment in that the ±1-order diffracted light beams from themask marks are focused on the wafer mark, that is, they are converged onone point on the wafer mark. To meet with this, imaginary mask 11-1 issituated at distance D ##EQU2## from mask 11.

More specifically, two alignment light beams, in the form of sphericalwaves, are illuminated to two mask marks 41-1 and 41-2, individually, ina manner such that they are collected in a incidence pupil of projectionlens 13. The alignment light beams are diffracted by the two mask marks,and two n-order diffracted light beams emerge to carry mask positioninformation based on their phase change. Two 0-order diffracted lightbeams enter the incidence pupil of lens 13. A -1-order diffracted lightbeam from mask mark 41-1 and a +1-order diffracted light beam from maskmark 41-2 are transferred to lens 13 as if they were light beamsemerging from b point of imaginary mask 11-1. The two ±1-orderdiffracted light beams take the form of plane waves after they aretransmitted through the projection lens. These two diffracted lightbeams, in the form of plane waves, are converged on one point on wafermark 42, whereupon they interfere with each other, thus forming aninterference fringe. This interference fringe is a moire pattern, aperiodic pattern, which depends on angle θ. Here θ is an angle valuehalf that of the angle formed between the ±1-order diffracted lightbeams converged on the wafer mark.

The two interfering ±1-order diffracted light beams are reflected bywafer mark 42 to be diffracted thereby again, and ±1-order re-diffractedlight beams emerge. This ±1-order re-diffracted light beams each other,are reflected perpendicularly by mark 42, and are then applied todetector 47 via mirror 51. Since the ±1-order re-diffracted light beamsare interference light beams, information on the respective positions ofthe mask and wafer can be obtained by detecting their intensity change.Thereafter, the mask and wafer are aligned with each other in the samemanner as in the first embodiment.

Thus, also in the second embodiment, the mask and wafer can be alignedwith each other even if the wavelength of the exposure light beam isextremely short, that is, even though the respective wavelengths of theexposure light beam and the alignment light beam are considerablydifferent. In contrast with the case of the conventional arrangement,moreover, return mirrors for correcting the optical paths of thediffracted light beams need not be disposed between the mask and wafer.Thus, the arrangement between the mask and wafer is simplified.

Further, according to this embodiment, the two ±1-order order diffractedlight beams from the mask marks are converged on one point on wafer mark42, where they interfere with each other. In contrast with the case ofthe first embodiment, therefore, the re-diffracted light beams from thewafer mark need not be superposed. Thus, there is no need of means forsuperposing the re-diffracted light beams.

In the first embodiment, furthermore, the wafer mark sometimes may beskewed, failing to be set horizontally, if the wafer is distorted. Sincethe two ±1-order diffracted light beams are separate from each otherwhen they are incident upon the wafer mark, the re-diffracted lightbeams from the wafer mark may possibly be reflected slantly, notvertically. Therefore, the alignment between the mask and wafer may besubject to an error. According to the second embodiment, however, thetwo ±1-order diffracted light beams are converged on one point of thewafer mark, so that there is no possibility of such an error. In thecase of the first embodiment, moreover, the re-diffracted light beamsmay be changed in phase if there is a difference between the ambienttemperatures around the -1- and +1-order re-diffracted light beams.Thus, the alignment may possibly be subject to an error. According tothe second embodiment, however, the re-diffracted light beams cannot bechanged in phase.

The following is a description of the ways of setting distance D betweenimaginary mask 11-1 and mask 11, the distance between mask marks 41-1and 41-2, and the pitch of the mask marks.

Let it be supposed that the inverse magnification of the projection lensfor the alignment light beam, indicative of the relation between therespective positions of the imaginary mask and the wafer mark, is β, theangle formed between the light beam emerging from point b on theimaginary mask and the optical axis of the projection lens is 81' andthe angle formed between the alignment light beam incident upon maskmarks 41-1 and 41-2 and the optical axis of the projection lens isθ_(R).

Thereupon, distance D between imaginary mask 11-1 and mask 11 anddistance 2r between two mask marks 41-1 and 41-2 have a correlation asfollows:

    r=Dtanθ.sub.1 =Dtanβθ.                    (1)

Pitch P of mask marks 41-1 and 41-2 are set as follows:

    nλ/P=sinθ.sub.R +sinθ.sub.1 =sinθ.sub.R +sinβθ,

where n is the order number of the diffracted light beams from the maskmarks, and λ is the wavelength of the alignment light beam. Thus, pitchP of the mask marks is set as if the ±1-order diffracted light beamsfrom the mask marks were ones emerging from the imaginary mask.

In the second embodiment, moreover, the two ±1-order order diffractedlight beams from the mask marks are focused on the wafer mark. However,these diffracted light beams need not be exactly focused, but only befocused within the depth of focusing. Also in this case, the ±1-orderdiffracted light beams interfere with each other, thereby forming aninterference fringe. Thus, the distance between the mask and wafer maybe somewhat varied.

Specifically, signal processing unit 48 may be arranged as follows. Forexample, it may be designed so that the mask or wafer 12 is oscillatedhorizontally at a predetermined frequency to modulate the alignmentlight beam, whereby the detection signals are synchronously demodulatedin accordance with the predetermined frequency. Alternatively, unit 48may be arranged so that the phase (or frequency) of the wavelength ofthe alignment light beam is modulated by mean of phase shift mechanism52, whereby the detection signals are synchronously demodulated inaccordance with the predetermined frequency.

Referring now to FIG. 7, a third embodiment of the present inventionwill be described.

This embodiment is a more specific version of the second embodiment.

In the third embodiment, two alignment light beams are applied to maskmarks 41-1 and 41-2 via lens 61 and condenser lens 62. In doing this,the alignment light beams are incident upon the mask marks so as to bedirected toward the center of the incidence pupil of projection lens 13(as indicated by broken line). Two ±1-order diffracted light beams frommarks 41-1 and 41-2 are transmitted through the incidence pupil to beconverged on wafer mark 42. As mentioned before, wafer mark 42 may be aone- (FIG. 8A) or two-dimensional diffraction grating (FIG. 8B) orcheckered diffraction grating (FIG. 8C). The best selection depends onthe operating conditions of a projection/exposure unit.

During transfer of a circuit pattern, for example, a mask and a wafersometimes may be aligned with each other by means of an alignment lightbeam. In this case, a two-dimensional diffraction grating (FIG. 8B) or acheckered diffraction grating (FIG. 8C) may be used as a wafer mark. Inthe case of the embodiment shown in FIG. 6, the intensity of there-diffracted light beams is more improved by employing the checkereddiffraction grating for the wafer mark. Thus, re-diffracted light beamsfrom the wafer mark are distributed in a two-dimensional manner. Amongthese re-diffracted light beams of the two-dimensional distribution,re-diffracted light beams of predetermined orders can be situated offthe optical path of the exposure light beam. If mirror 51 is located sothat the predetermined-order re-diffracted light beams can be detected,therefore, the exposure light beam cannot be intercepted by the mirror.

Whether wafer mark 42 is illuminated by the exposure light beam so thata resist of mark 42 is separated, or whether the wafer mark is notilluminated by the exposure light beam so that the resist remainsunseparated, depends on a user's selection. In other words, the user canselect the separation or retention of wafer mark 42 by forming achromium-free window at point e of mask 11 which is conjugate to thewafer mark, or by depositing chromium to point e to intercept theexposure light beam.

FIG. 9 shows modifications of the detecting means and the signalprocessing means.

In FIG. 9, mask 11 has four continuously arranged marks A, B, C and D,as mask marks 41-1, and four continuously arranged marks a, b, c and d,as mask marks 41-2. It is necessary only that marks A to D and a to d bearranged on a dicing line outside a circuit pattern. Marks A, B, C and Dare spaced at a predetermined distance from marks a, b, c and d,respectively. In wafer 12, on the other, four marks Aa, Bb, Cc and Ddare continuously arranged as wafer marks. Thus, when the alignment lightbeam is applied to marks A and a of the wafer, diffracted light beamsfrom these marks are transferred to mark Aa of the wafer to bediffracted thereby. Likewise, when the alignment light beam is appliedto marks B and b (or C and c, or D and d), the diffracted light beamsare transferred to mark Bb (or Cc or Dd) of the wafer.

The following three methods are available as signal processing methodsusing the marks described above.

According to the first method, the alignment beams incident on marks Aand a are slightly oscillated in the alignment direction. Since thediffracted light reflected by the wafer also oscillates, a modulateddetection signal is obtained as a result of the oscillation. Thistechnique is similar to that used in the signal processings performed inan ordinary photoelectric microscope.

According to a second method, the alignment light beam is appliedalternately to marks A and a and to marks B and b. As a result,re-diffracted light beams emerge alternately from marks Aa and Bb of thewafer. The pitch of mark Aa of the wafer is deviated from that of markBb by a quarter pitch. Therefore, the re-diffracted light beams frommarks Aa and Bb are different in diffraction angle, so that they aredetected independently of each other, or alternately. Then, thedifference between these two re-diffracted light beams is calculated,and the mask and wafer are aligned so that the difference is zero.

According to the third method, the alignment beams are not oscillated.Instead, the right and left alignment beams are made to have a frequencydifference of ω by means of frequency modulator 52, as is shown in FIG.6. Since the re-interfered light coming from wafer mark 42 produces abeat signal resulting from frequency difference ω, the positionadjustment can be performed by measuring a phase change in the beatsignal. (This method is generally referred to as an optical heterodynemethod.)

When manufacturing a semiconductor device, a plurality of circuitpatterns are transferred successively to one wafer. Accordingly, themask and wafer must be aligned with each other every time a pattern istransferred. Thus, a number of alignment marks should be formed on thewafer. As shown in FIG. 9, for example, marks Aa and Bb of the wafer areused for the alignment for the transfer of a first layer, and marks Ccand Dd are used for the alignment for the transfer of a second layer.Let us suppose, however, that the alignment light beam is applied tomarks A and a and marks B and b of the mask at the time of the transferof the first layer. In this case, individual ±1-order diffracted lightbeams from marks A, a, B and b are transferred to marks Aa and Bb of thewafer. Possibly, however, 0-order diffracted light beams from marks A, aB and b may be illuminated to mark Cc or Dd of the wafer. In such acase, the 0-order diffracted light beams are reflected by mark Cc o Ddto be incident upon detector 47, so that the alignment is subject to anerror. This error can be prevented by setting distance 2r between themask marks relatively large so that angle θ at which the diffractedlight beams from the mask marks are incident upon the wafer can be setrelatively wide.

In the embodiments described above, the mirror is disposed between themask and the wafer, whereby the re-diffracted light beams from the wafermark are guided to the detector. Alternatively, however, as shown inFIG. 3 the mirror may be disposed in a manner such that there-diffracted light beams from the wafer mark are guided to the detectorafter they are transferred to above the mask. Moreover, two wafer marksand one mask mark may be arranged on the wafer and the mask,respectively. As shown in FIG. 7, furthermore, the two mask marks may bein the form of one diffraction grating which extends relatively long.

What is claimed is:
 1. A method for aligning first and second objectswith each other, a projection lens disposed between the first and secondobjects, the first object having one first diffraction point, the secondobject having two second diffraction points spaced at a predetermineddistance, said method comprising the steps of:directing a light beamemitted from a light source to the first diffraction point so that twodiffracted light beams emerge from the first diffraction point;transferring the two diffracted light beams through the projection lenstoward the two second diffraction points, the two diffracted light beamsbeing converged by the projection lens and then being incident on thetwo second diffraction points, so that two re-diffracted light beamsemerge respectively from the two second diffraction points; detectingthe two re-diffracted light beams, thereby obtaining a detection signalwhich corresponds to a displacement between the first and secondobjects; and aligning the first and second objects in accordance withthe displacement.
 2. A method according to claim 1, wherein respectiveoptical axes of the two diffracted light beams, which are directed in anadvancing direction of the diffracted light beams, intersect each otherat an intersection point at a predetermined distance from the secondobject.
 3. A method for aligning first and second objects with eachother, a projection lens disposed between the first and second objects,the first object having one first diffraction point, the second objecthaving two second diffraction points spaced at a predetermined distance,said method comprising the steps of:directing a light beam emitted froma light source to the first diffraction point, so that two diffractedlight beams emerge from the first diffraction point; transferring thetwo diffracted light beams through the projection lens toward the twosecond diffraction points, the two diffracted light beams beingconverged by the projection lens and then being incident on the twosecond diffraction points, so that two re-diffracted light beams emergerespectively from the two second diffraction points; interfering the twore-diffracted light beams with each other, thereby generating a lightbeat in the two re-diffracted light beams; detecting the light beat,thereby obtaining a phase difference of the detected beat with respectto a reference beat, and the phase difference corresponding to adisplacement between the first and second objects; and aligning thefirst and second objects in accordance with the displacement.
 4. Amethod according to claim 3, wherein respective optical axes of the twodiffracted light beams, which are directed in an advancing direction ofthe diffracted light beams, intersect each other at an intersectionpoint at a predetermined distance from the second object.
 5. A methodaccording to claim 3, wherein respective directing step includes a stepfor generating two light beams of different frequencies and a step fordirecting the two light beams onto the first diffraction point.
 6. Amethod for aligning first and second objects with each other, aprojection lens disposed between the first and second objects, the firstobject having two first diffraction points spaced at a predetermineddistance, the second object having two second diffraction points spacedat a predetermined distance, said method comprising the stepsof:directing two light beams emitted from a light source to the twofirst diffraction points so that two diffracted light beams emergerespectively from the two first diffraction points; transferring the twodiffracted light beams through the projection lens toward the two seconddiffraction points, the two diffracted light beams being converged bythe projection lens and then being incident on the two seconddiffraction points, so that two re-diffracted light beams emergerespectively from the two second diffraction points; interfering the twore-diffracted light beams with each other, thereby generating a lightbeat in the two re-diffracted light beams; detecting the light beat,thereby obtaining a phase difference of the detected beat with respectto a reference beat, the phase difference corresponding to adisplacement between the first and second objects; and aligning thefirst and second objects in accordance with the displacement.
 7. Amethod according to claim 6, wherein respective optical axes of the twodiffracted light beams, which are directed opposite to an advancingdirection of the two diffracted light beams, intersect each other at aninspection point at a predetermined distance from the first object.
 8. Amethod according to claim 6, wherein respective optical axes of the twodiffracted light beams, which are directed in an advancing direction ofthe diffracted light beams, intersect each other at an intersectionpoint at a predetermined distance from the second object.
 9. A methodaccording to claim 6, wherein said directing step includes a step forgenerating two light beams of different frequencies and a step forindividually directing the two light beams onto the two firstdiffraction points.
 10. A method for aligning first and second objectswith each other, a projection lens disposed between the first and secondobjects, the first object having two first diffraction points spaced ata predetermined distance, the second object having one seconddiffraction point, said method comprising the steps of:directing twolight beams emitted from a light source to the two first diffractionpoints, so that two diffracted light beams emerge from the two firstdiffraction points; transferring the two diffracted light beams throughthe projection lens toward the second diffraction points, the twodiffracted light beams being forced on the second diffraction point bythe projection lens, so that one re-diffracted and interfered light beamemerges from the second diffraction point; detecting a light beatgenerated in the re-diffracted and interfered light beam, therebyobtaining a phase difference of the detected beat with respect to areference beat, the phase difference corresponding to a displacementbetween the first and second objects; and aligning the first and secondobjects in accordance with the displacement.
 11. A method according toclaim 10, wherein respective optical axes of the two diffracted lightbeams, which are directed opposite to an advancing direction of the twodiffracted light beams, intersect each other at an intersection point ata predetermined distance from the first object.
 12. A method accordingto claim 10, wherein said directing step includes a step for generatingtwo light beams of different frequencies and a step for individuallydirecting the two light beams onto the two first diffraction points. 13.An apparatus for aligning first and second objects with each other, aprojection lens disposed between the first and second objects, the firstobject having one first diffraction point, the second object having twosecond diffraction points spaced at predetermined distance, comprising:alight source for emitting an alignment light beam; means for directingthe alignment light beam to the first diffraction point, so that twodiffracted light beams emerge from the first diffraction point, the twodiffracted light beams passing through the projection lens toward thetwo second diffraction points, being converted by the projection lensand then being incident on the two second diffraction points, so thattwo re-diffracted light beams emerge respectively from the two seconddiffraction points; means for detecting the re-diffracted light beamsand generating a detection signal which corresponds to a displacementbetween the first and second objects; and means for adjusting said firstand second objects relative to each other in response to the detectionsignal, thereby aligning said first and second objects with each other.14. An apparatus according to claim 13, wherein respective optical axesof the two diffracted light beams, which are directed in an advancingdirection of the diffracted light beam, intersect each other at anintersection point at a predetermined distance from the second object.15. An apparatus for aligning first and second objects with each other,a projection lens disposed between the first and second objects, thefirst object having one first diffraction point, the second objecthaving two second diffraction points spaced at a predetermined distance,comprising:a light source for emitting an alignment light beam; meansfor directing the alignment light beam to the first diffraction point,so that two diffracted light beams emerge from the first diffractionpoint, the two diffracted light beams passing through the projectionlens toward the two second diffraction points, being converted by theprojection lens and then being incident on the two second diffractionpoints, so that two re-diffracted light beams emerge respectively fromthe two second diffraction points; means for receiving the re-diffractedlight beams and producing a beat signal which is generated byinterfering the re-diffracted light beams with each other, therebyobtaining a phase difference of the heat signal with respect to areverence beat signal, the phase difference corresponding to adisplacement between the first and second objects; and means foradjusting said first and second objects relative to each other inaccordance with the displacement, thereby aligning said first and secondobjects with each other.
 16. An apparatus according to claim 15, whereinrespective optical axes of the two diffracted light beams, which aredirected in an advancing direction of the diffracted light beam,intersect each other at an intersection point at a predetermineddistance from the second object.
 17. An apparatus according to claim 15,wherein said alignment light beam directed on the first diffractionpoint includes two beams of different frequencies.
 18. An apparatus foraligning first and second objects with each other, a projection lensdisposed between the first and second objects, the first object havingtwo first diffraction points, spaced at a predetermined distance, thesecond object having two second diffraction points spaced at apredetermined distance, comprising:means for emitting two alignmentlight beams; means for directing the two alignment light beams to thetwo first diffraction points, so that two diffracted light beams emergerespectively from the two first diffraction points, the two diffractedlight beams passing through the projection lens toward the two seconddiffraction points, being converged by the projection lens and thenbeing incident on the two second diffraction points, so that twore-diffracted light beams emerge respectively from the two seconddiffraction points; means for receiving the re-diffracted light beamsand producing a beat signal which is generated by interfering there-diffracted light beams with each other, thereby obtaining a phasedifference of the heat signal with respect to a reverence beat signal,the phase difference corresponding to a displacement between the firstand second objects; and means for adjusting said first and secondobjects relative to each other in accordance with the displacement,thereby aligning said first and second objects with each other.
 19. Anapparatus according to claim 18, wherein respective optical axes of thetwo diffracted light beams, which are directed opposite to an advancingdirection of the diffracted light beam, intersect each other at anintersection point at a predetermined distance from the first object.20. An apparatus according to claim 18, wherein respective optical axesof the two diffracted light beams, which are directed in an advancingdirection of the diffracted light beam, intersect each other at anintersection point at a predetermined distance from the second object.21. The apparatus according to claim 18, wherein each of said alignmentlight beams directed on the two second diffraction points includes twobeam having different frequencies.
 22. An apparatus for aligning firstand second objects with each other, a projection lens disposed betweenthe first and second objects, the first object having two firstdiffraction points, spaced at a predetermined distance, the secondobject having one second diffraction point comprising:means for emittingtwo alignment light beams; means for directing the two alignment lightbeams to the two first diffraction points, so that two diffracted lightbeams emerge respectively from the two first diffraction points, the twodiffracted and interfered light beams passing through the projectionlens toward the second diffraction point, and forced on the seconddiffraction point by the projection lens, so that one re-diffractedlight beam emerges from the second diffraction point; means forreceiving the re-diffracted light beam and producing a beat signal whichis generated by interfering light beams with each other, therebyobtaining a phase difference of the heat signal with respect to areverence beat signal, the phase difference corresponding to adisplacement between the first and second objects; and means foradjusting said first and second objects relative to each other inaccordance with the displacement, thereby aligning said first and secondobjects with each other.
 23. An apparatus according to claim 22, whereinrespective optical axes of the two diffracted light beams, which aredirected opposite to an advancing direction of the two diffracted lightbeams, intersect each other at an intersection point at a predetermineddistance from the first object.
 24. An apparatus according to claim 22,wherein each of said alignment light beams directed on the two firstdiffraction points includes two beams having different frequencies.